1. Field of the Invention
This invention relates to a process for introducing fine solid particles into fluid streams under accurate control. The solid particles are contained in a foam for mixture with a fluid jet stream. This invention can be advantageously used to generate abrasive fluid jet streams having material-cutting capabilities heretofore unobtainable.
2. Description of the Prior Art
Many materials encountered in industry are very hard and tough making cutting, drilling and shaping of these materials difficult with the requirements of special tools, techniques and skills. Tools and methods currently available for cutting these materials have shortcomings and limitations that need to be reduced or eliminated. Further, the present consideration of energy consumption and efficiency places new emphasis on improved tools and methods for cutting such materials.
The usual method for cutting steel plate involves the use of mechanical or thermal tools that have undesirable characteristics such as slow speed, tool wear, poor edge quality, alterations of metallurgical properties, and fire hazards.
Concrete, rock and minerals are also difficult to cut, drill or break because of their mineral compositions and abrasive nature. The presence of steel reinforcing rods in reinforced concrete further increases the difficulties. Currently, saws and drills equipped with carbide or diamond-studded cutting edges are the only workable tools for cutting or drilling these materials. These tools have recognized limitations, such as rapid wear of cutting edges; ability to cut only shapes and patterns allowed by the geometry of the cutting edges; expense of diamond-studded edges; necessity to maintain a large tool inventory to meet the requirements of various jobs; slow operation due to hardness and abrasiveness of material to be cut; and the cutting can be very noisy, dusty and fatiguing to operating personnel. Breaking concrete and rock is usually achieved by use of the commonly available jackhammers which are grossly inadequate. Thus, removing a large volume of concrete or rock without using explosives can be a slow, expensive and energy consuming operation.
There are also difficulties associated with cutting high strength plastics and composites in production plants. For example, graphite and Kevlar fiber reinforced laminates are difficult to cut because of the abrasive nature of these fibers and the need to avoid delamination in cutting. In some operations, the work pieces are three dimensional wherein cutting or trimming must follow the surface contours and the work pieces must be rigid enough and/or fastened to withstand the cutting forces. The development of new engineering materials has imposed new requirements for cutting tools and techniques. The need for new and more effective cutting methods has become very urgent and continuous efforts have been devoted in recent years to the development of better cutting methods.
One of the relatively new methods for cutting and breaking materials utilizes a stream of water traveling at high velocity in a water jet. The water jet is already being employed to cut a wide variety of materials, including synthetic polymers, leather, paper products, fiberglass, asbestos and textiles. Description of the water jet apparatus and its applications are found in the following publications: H. D. Harris and W. H. Brierley, "Application of Water Jet Cutting", Paper G-1, 1st International Symposium on Jet Cutting Technology, Coventry, U.K., April 1972; E. N. Leslie, "Application of the Water Jet to Automated Cutting in the Shoe Industry", Paper F-3, 3rd International Symposium on Jet Cutting Technology, Chicago, May 1976; and T. J. Labus, "Cutting and Drilling of Composites Using High Pressure Water Jets", Paper G-2, 4th International Symposium on Jet Cutting Technology, Canterbury, U.K., April 1978. In the apparatus and methods described, water is pressurized to a level as high as 60,000 psi and ejected through a small orifice to generate a high velocity, substantially coherent water jet. Such a water jet possesses high kinetic energy and can cleanly cut many materials. There are many advantages for using a water jet to cut materials, including absence of tool wear, absence of direct tool contact with the target material, and minimum dust problems. In some applications, the speed of cutting is also increased and the quality of cut improved by employing the water jet method.
The water jet cutting method has not been used widely due primarily to its high equipment cost resulting from the high fluid pressure involved, high energy consumption and the inability to satisfactorily cut hard and tough materials, such as concrete, rock, glass, hard plastics and metals. Attempts have been made to cut such materials with a water jet by increasing the water pressure and thus the power input to a very high level. These attempts have not been satisfactory due to the cost of the equipment escalating drastically with the increased pressure and power while the quality of cutting has not been improved proportionally. For example, attempts to cut concrete with a water jet having power input in excess of 200 hp and water pressure greater than 50,000 psi have not been a complete success as concrete and aggregates tend to spall rather than being cut cleanly and the debris generated by the high pressure water jet settles in the cut volume hampering the cutting process. The application of high pressure water jets to cut rock and concrete has been discussed in many publications including: L. H. McCurrich and R. D. Browne, "Application of Water Jet Cutting Technology to Cement Grouts and Concrete", Paper G-7, 1st International Symposium on Jet Cutting Technology, Coventry, U.K., April 1972; A. G. Norsworthy, U. H. Mohaupt and D. J. Burns, "Concrete Slotting with Continuous Water Jets at Pressures up to 483 MPa", Paper G-3, 2nd International Symposium on Jet Cutting Technology, Cambridge, U.K., April 1974; and T. J. Labus and J. A. Hilaris, "Highway Maintenance Application of Jet Cutting Technology", Paper G-1, 4th International Symposium on Jet Cutting Technology, Canterbury, U.K., April, 1978. A high pressure pulsed water jet apparatus and process is taught by U.S. Pat. No. 4,074,858.
Abrasive particles propelled by compressed air have been used to cut many hard materials. This method can be quite effective when the abrasive particles are accelerated to high velocity and ejected through a suitable nozzle. However, the difficulty in containing the particles and dust during cutting operation prohibits its use in large scale material cutting. Currently, air-propelled abrasive powders are used for deburring metals and for surface preparation of materials where a hood or an enclosure can be employed to contain the dust. A wide variety of abrasive powders, such as silicon carbide, aluminum oxide, garnet, glass beads and silica sand are used for such applications.
The combination of solid particles with a fluid jet has been employed for several uses. For example, U.S. Pat. No. 2,821,396 teaches solid particles in an air or steam injector as an attrition impact pulverizer; U.S. Pat. No. 3,424,386 teaches mixing of granular solids with a liquid for use in sandblasting; U.S. Pat. Nos. 3,972,150 and 3,994,097 teach water jets of particulate abrasive for cleaning with water pressures under 5,000 psi; U.S. Pat. No. 4,080,762 teaches a fluid-abrasive jet for paint removal with fluid pressures up to 30,000 psi; and U.S. Pat. No. 4,125,969 teaches a wet abrasion blast cleaning apparatus and method utilizing soluble abrasive materials. These patents show that combining abrasive particles with water jets have not produced an abrasive water jet capable of cutting hard materials. The jets generated by the devices taught by these patents can at best clean and blast the surface of hard materials. The prior devices fail in achieving cutting capability of hard materials primarily because the devices fail to generate a sufficiently high velocity and sufficiently coherent water jet; and fail to mix the abrasive particles with the high velocity water stream in sufficient quantity.
U.S. Pat. Nos. 3,424,386, 3,972,150, 4,080,762 and 4,125,969 all teach the abrasive (sand) stream to be in the central portion of the nozzle while the pressurized fluid is introduced into the peripheral area surrounding the central sand stream. A ring orifice plate or disk such as employed in the U.S. Pat. Nos. 3,424,386, 4,080,762 and 4,125,969 to provide the fluid jets around the sand stream has many disadvantages including: the introduction of pressurized fluid tangentially into a nozzle a short distance above the orifice disk is not conducive to the generation of a coherent fluid jet due to flow disturbances upstream of the orifices; sand in the central portion of a nozzle creates an abrasive environment that can weaken the interior wall of the annular fluid chamber without being detected; pressurized fluid in the outer annular space results in a nozzle that is very large in dimensions as both interior and exterior walls must be sized to accommodate the fluid pressure; and sealing the annular orifice disk can be very troublesome. The U.S. Pat. No. 3,994,097 teaches a centrally located water jet while sand is fed into a nozzle chamber through a single sand passageway. The sand is forced into the water jet by passage through a conical nozzle. This patent recognizes abrasion problems within the nozzle and the necessity of exact alignment. These problems would be intensified at higher pressures. All of these patents teach mixing abrasive into water by (1) intercepting an abrasive stream with water jets, and (2) forcing abrasives, water and air through a conical nozzle, without concern of fluid actions.
The prior art devices have generally utilized compressed air to deliver the abrasive particles to a nozzle in which the particles are mixed with the water stream. It is desirable, however, for the particles to be wetted by water before they are to be most effectively mixed with the water. Further, if the water stream is coherent and is traveling at high speed, the conditions are not favorable for the air propelled particles to be mixed into the water stream. At best, some particles are carried away by the water droplets formed around the coherent core of the water stream. The introduction of abrasive particles would be significantly improved if the water jet is made to disperse into droplet form, however, the resultant abrasive water jet would be weak and incapable of cutting hard materials.
The transporting of abrasive particles by compressed air or gas also has other undesirable characteristics. Since abrasive particles are generally heavy, the air flow must be sufficiently turbulent to move the particles, otherwise the particles will settle and block the passage. The air or gas must be dry to avoid agglomeration of particles and resulting blockage of the passage. Further, erosion of tubings, hoses and fittings by the abrasive particles is a common problem. The air or gas used to propel the abrasive particles can interfere with the formation of a coherent abrasive water jet and result in a dust problem as some abrasive particles will escape with the air or gas without being mixed with the water.
A possible alternative approach of transporting abrasive particles to the nozzle is to convert the abrasives to a slurry as taught by U.S. Pat. No. 3,972,150. This abrasive slurry is then pumped into a nozzle and mixed with the water jet. One problem of this approach is that the slurry must be mixed into the water jet, the mixing of which can consume a significant amount of the water jet's kinetic energy as the slurry rather than the individual abrasive particles must be accelerated to the water jet velocity. Such loss of water jet energy can be particularly severe if the abrasive slurry is viscous. These problems are increased by the fact that high viscosity may be necessary in formulating such an abrasive slurry, if settlement of the particles is to be avoided.